Various underdrain systems have been developed for filter systems that filter water and wastewater. One commonly used underdrain system is a monolithic false bottom underdrain system. This underdrain system typically includes a cast in place concrete structure that overlays the structural floor or true bottom of the filter housing or compartment. The concrete structure includes a plurality of cone shaped depressions formed herein. “Cone shaped” as used herein includes within its definition pyramidal shaped depressions. A thimble is disposed at the lowermost point of the cone shaped depressions. Fluid flows through the thimble and corresponding cone shaped depressions during the various cycles of operation of a filter including the filtration mode and backwashing mode. Fourteen porcelain spheres are commonly disposed in each of the cone shaped depressions. Five of the porcelain spheres are approximately 3 inches in size, one of the porcelain spheres is approximately 1 ⅜ inches in size and the eight remaining porcelain spheres are approximately 1 ¼ inches in size. One of the 3 inch porcelain spheres is disposed directly over a porcelain thimble positioned in the bottom of the cone shaped depressions. The remaining four 3 inch balls are disposed directly above the porcelain sphere positioned over the thimble. The 1 ⅜ inch porcelain sphere is located in the center of the four 3 inch porcelain spheres. The remaining eight 1 ¼ inch porcelain spheres are positioned on the outer periphery of the four 3 inch porcelain spheres. The porcelain balls are expensive and time-consuming to install. Further, it is important to insure that the balls are spherical to insure that the balls do not spin owing to the flow of water passing the balls. This spinning can cause undue wear on the walls of the depressions. The porcelain balls are also subject to becoming dislodged from the corresponding depression. This can lead to significant problems with the operation of the filter.
Another commonly employed underdrain system is a pre-cast wheeler bottom. This type of underdrain system is typically shipped in blocks of specific sizes such as 2′×2′ as opposed to a monolithic false bottom underdrain system as previously described. The pre-cast blocks are positioned on concrete walls in the filter housing or compartment. A plurality of depressions are formed in the pre-cast blocks. A plurality of porcelain spheres are disposed in each of the depressions. This underdrain system suffers from similar drawbacks to the monolithic false bottom underdrain systems.
A pyramidal shaped liner has been employed in the depressions of the concrete bottoms where the walls have deteriorated. However, this pyramidal shaped liner formed from plastic only addressed the structural deficiency of the walls and still required the use of numerous porcelain spheres or balls.
U.S. Pat. No. 6,190,568 discloses retrofitting a monolithic false bottom filter underdrain system by replacing the porcelain balls with one or more porous plates. The primary if not exclusive reason for substituting the porcelain balls with a porous plate stated in U.S. Pat. No. 6,190,568 is to obviate one or more gravel support layers that are sometimes used with monolithic false bottom underdrain systems. The porous plates are permanently anchored to the concrete bottom of the monolithic false bottom underdrain system. Specifically, U.S. Pat. No. 6,190,568 teaches permanently anchoring the porous plates to the concrete bottom by embedding an anchor in the concrete bottom.
There are numerous disadvantages to the retrofit underdrain system disclosed in U.S. Pat. No. 6,190,568. One principal disadvantage is that the porcelain balls are replaced with an inferior fluid distributor. Specifically, unlike the porcelain balls, porous plates are not particularly good distributors. This causes significant problems during operation of the filter. For example, fluid directed through a porous plate is more likely to pass through the central portion of the porous plate rather than be evenly distributed over the entire surface area of the porous plate. This results in maldistribution of the fluid in the filter bed. The retrofit underdrain system disclosed in U.S. Pat. No. 6,190,568 employs multiple layers of porous plates to achieve better distribution. However, adequate distribution is not achieved owing to limitations on the distribution capabilities of the porous plate. Further, using multiple porous plates unnecessarily increases the cost of the retrofit underdrain system. The retrofit underdrain system disclosed in U.S. Pat. No. 6,190,568 is unnecessarily difficult and time consuming to install. Specifically, embedding the anchoring members in concrete to permanently secure the porous plate to the concrete bottom is a difficult and time-consuming process due in part to the presence of reinforcing steel used in the concrete. The difficulty in the installation process can lead to structural failure. Moreover, the porous plate cannot be readily replaced when necessary owing to its permanent attachment to the concrete bottom.